Electrographic apparatus

ABSTRACT

A combination powdering and scavenging device for electrographic printing and/or display apparatus. Powder is transferred between a powder reservoir and at least one movable recording surface in a manner which provides for a lossless powder system for the apparatus. Powder transfer is accomplished by a readily detachable device comprising a pair of movable surfaces which are spaced apart in the direction of movement of the recording surface and which are parallel thereto. With one surface for powdering and the other for scavenging at least one of the pair is permanently magnetized or charged electrostatically and partially submerged in powder contained in the reservoir.

United States Patent [151 3,636,926 Hodges 1 Jan. 25, 1972 [541 ELECTROGRAPHIC APPARATUS 3,345,944 10/1967 Simmons ..1 18/637 x 3,383,699 5 1968 L k ..ll8 37 [72] Inventor: Robert James Hodges, Cheshunt, England apms l /6 UX [73] Assignee: International Standard Electric Corpora- Primary ExaminerMervin stein on, New York, Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Percy P. Lantzy, Philip M. Bolton, Isidore Togut and Charles L. John- [22] Filed: Apr. 1, 1969 SonJL 21 A l.No.: 812 230 1 pp 57 ABSTRACT 52 us. 01 ..ll8/637, 118/257 118/258 A cmbinatin Pwdeflng and scavenging device elect [51] Int. Cl. ..G63g 13/08 graphic priming and/or display apparatus Powder is trans" [58] Field f Search 118/255 257 258 623 ferred between a powder reservoir and at least one movable 3 recording surface in a manner which provides for a lossless powder system for the apparatus. Powder transfer is accom- [56] Reierences Cited plished by a readily detachable device comprising a pair of movable surfaces which are spaced apart in the direction of UNITED STATES PATENTS v movement of the recording surface and which are parallel thereto. With one surface for powdering and the other for 3/1957 Young ..1 18/257 X scavenging at least one of the p is permanently magnetized 290l374 g d l 7/17 5 or charged electrostatically and partially submerged in un ac powder contained in the reservoir.

29 Claims, 6 Drawing Figures PATENTED masmz 3 636 926 noaznr J. HOD6$ A Home y I PATENTED Jmesmz lnvenlor ROBERT J. HODQES A Home y PATENTEU JAN25 I972 SHEEI 3 [IF 3 lnvenlor 2005/17 J. l-looqEs Attorney ELECTROGRAPIIIC APPARATUS BACKGROUND OF THE INVENTION The invention relates to electrographic printing and/or display apparatus.

Electrographic printing and/or display apparatus is defined as apparatus of the kind wherein the recording surface of either a print drum or band is selectively magnetized or electrostatically charged to form a pattern thereon representative of information contained in a signal applied to the apparatus, and wherein the print drum or band is passed through or relative to a powder applicator containing a powder that is attracted to the electromagnetically or electrostatically fonned pattern, thereby producing a powder pattern which may be viewed at a display position and/or transferred to a permanent record by printing means which form part of the apparatus. When it is required to change the stored information on the print drum or band it is necessary to scavenge the powder from the recording surface of the drum or band and then erase the image in preparing the recording surface for recording.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an electrographic printing and/or display apparatus of the kind as hereinbefore defined which utilizes a combined powder application and scavenging device.

The invention provides an electrographic printing and/or display apparatus of the kind as hereinbefore defined including a combined powder applicator and scavenging device wherein powder is transferred between a powder reservoir and at least one movable recording surface of the apparatus by a pair of movable surfaces which are spaced apart in the direction of movement of the recording surface and which are parallel with the recording surface, at least one of the pair of movable surfaces being either permanently magnetized or charged electrostatically and partially submerged in powder contained in the reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other features according to the invention will be better understood from the following description with reference to the accompanying drawings, in which:

FIGS. 1A and 1B diagrammatically illustrate a front elevation and a plan view of part respectively of an electrographic printing and/or display apparatus of the kind as hereinbefore defined having a horizontal print band and a combined powder applicator and scavenging device;

FIGS. 2A and 2B diagrammatically illustrate a front elevation and a plan view respectively of another embodiment of the combined powder applicator and scavenging device;

FIG. 3 diagrammatically illustrates a section through X-X of the combined powder applicator and scavenging device shown in the drawing according to FIGS. 2A; and

FIG. 4 diagrammatically illustrates a sectional side elevation of part of an electrographic printing and/or display apparatus of the kind as hereinbefore defined having a vertical print band and a modified arrangement of the combined powder applicator and scavenging device shown in the drawings according to FIGS. 2A and 2B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1A and 1B, a front elevation and a plan view of part of an electrographic printing and/or display apparatus of the kind as hereinbefore defined, having a horizontal print band, and a combined powder applicator and scavenging device, are respectively diagrammatically illustrated therein. It will be assumed, by way of example, that the pattern on the recording surface of the print band is formed electromagnetically in which case the horizontal print band will be a magnetic print band. The combined powder applicator and scavenging device comprises two discs I and 5 which are respectively rotatably mounted on the sidewall of a fluid powder bed 6 by means of coaxial shafts 4 and 2 and which are driven by a drive motor unit 10 in the direction of the arrows A via the belt drives 7 and 8. The belt drive 7 is located in an annular groove 13 provided in the shaft 4 and one of the two annular grooves 14 provided in the shaft 2 while the belt drive 8 is located in the other of the two annular grooves 14 and an annular groove 15 provided in the drive shaft 9 of the motor unit 10.

The axis of rotation of the discs 1 and 5 is at right angles to the magnetized surface 3 of the horizontal magnetic print band which is being driven by means not shown in the drawings in the direction of the arrow B, and approximately half of the disc surfaces 47 and 48 are at all times during the operation of the apparatus in close proximity to, and scanning the recording surface 3.

The disc 5, which is the first one the recording surface 3 encounters when the apparatus is operative, is the scavenging disc, and when a permanent erase mode of operation is required, i.e., when a magnetic pattern formed by a recording head assembly 12 on part of the recording surface 3 is to be erased before that part of the recording surface 3 again encounters the recording head assembly 12, the disc 5 is of a nonmagnetic material and the surface 48 thereof is covered with a thin sheet of permanently magnetized material, or the disc 5 itself may be provided by a permanent disc magnet. In either case the magnetization of the surface 48 is arranged such that it takes the form of a series of striations which preferably take the form of radials from the center of the disc 5.

When a permanent erase mode of operation is not required, then the surface 48 of the disc 5 is covered with a nonmagnetic material which will be arranged such that it will afford keying for the magnetic powder on the recording surface 3 to the surface 48. Alternatively, the disc 5 may itself be made from a nonmagnetic material and the surface 48 could be roughened for example such that it affords keying for the magnetic powder on the recording surface 3. If a selective or permanent erase mode of operation is required when a nonmagnetic surface 48 is utilized, then it would be necessary to provide an erase head 16, for example, an electromagnet, so designed as to produce magnetic flux parallel to the recording surface 3 in order to erase previously recorded magnetic patterns when required.

In operation, magnetic powder on the moving recording surface 3 is attracted to or collected by the rotating surface 48 of the disc 5 and is carried into the fluid powder bed 6 into which the lower half of the disc 5 is immersed. As each section of the disc 5 emerges from the fluidized magnetic powder 49 which is contained within the bed 6 and supported on the diaphragm 17, the powder thereon is removed by a stripper or dressing blade 11 which is secured to the sidewall of the fluid bed 6. Alternatively, the stripper or dressing blade 11 may be replaced by a brush attachment. In the case of selective erasure, the erase head 16 would be repositioned such that it is directly opposite and behind the disc 5; therefore, only when the erase head 16 is energized will the magnetized recording surface 3 be erased and scavenged.

When the surface 48 of the scavenging disc 5 is pennanently magnetized it may still be necessary, in order to ensure that after the erase operation there is a completely uniform flux pattern on the recording surface 3, to provide an erase head 16. The need for the erase head depends mainly on the mode of operation of the recording head assembly 12, i.e., whether it is in or out of contact with the surface of the magnetic print band opposite the recording surface 3, and the direction of magnetization of the recording surface 3 in relation to the magnetization of the surface 48.

After passing the scavenging disc 5, and in certain applications the erase head 16, the recording surface 3 is passed relative to the recording head assembly 12 and is selectively magnetized to form a magnetic pattern thereon representative of information contained in a signal applied to the apparatus.

The recording surface 3 with the magnetic pattern thereon then encounters the disc l the lower half of which is immersed in the magnetic powder 49' within the fluid bed 6. The construction of the disc l is the same as the permanent erase scavenger disc 5, Le, it is either a permanent disc magnet or is provided with a permanently magnetized surface 47, the magnetization in each case taking the form of radial striations. The magnetic powder d9 within the fluid bed 6 is attracted to the disc l as it is rotated therethrough to provide on the surface d7 an evenly distributed layer of the magnetic powder. The permanently magnetized surface 47 is separated from the recording surface 3 by an amount such that consistent powder transfer to the magnetic pattern on the recording surface 3 is obtained, i.e., the recording surface 3 is arranged to just contact the outer surface of the evenly distributed layer of printing powder on the surface 47, and such that the magnetized surface 47 does not have a demagnetizing effect on the latent magnetic image. The strength of the magnetic field of the surface 47 therefore determines the separation distance between the recording surface 3 and the surface 47, as a stronger field will yield a greater build up of the magnetic powder 49 and therefore the gap between the transfer surface 47 and the recording surface 3 will need to be larger in order to ensure the recording surface 3 just contacts the outer surface of the layer of magnetic powder on the surface 47. Therefore, a stronger magnetic field will have less demagnetization effect on the magnetic patternv After passing the disc 1, the magnetic pattern on the recording surface 3 will have been transformed to a powder pattern thereby forming a two-dimensional visible image which is suitable for viewing at a display position of the apparatus and/or transferring to a permanent record at a print station of the apparatus.

The magnetic print band may be similar to that of magnetic tapes used in conventional recording apparatus, for example a support band of an insulating and nonmagnetic material such as a polymer film, coated on the surface 3 with a thin film of magnetic material such as iron oxide or a nickel cobalt alloy. The color of the printing powder is preferably dark to provide the highest possible contrast with the surface 3; it is preferred to provide on the surface 3 a white, matt silver or reflecting coating which need only be a few microns thick, in order to increase the contrast and hence the legibility of the powdered image or images. It will be seen from the drawings that the magnetic recording head assembly 12 operates on the opposite surface of the magnetic print band from the recording surface 3 to which the magnetic powder is applied. This arrangement is preferred in order to reduce the possibility of abrasion of the contrasting surface by the recording head assembly 12, but since the magnetic coating on the film must then be on the side remote from the magnetic recording head assembly T2, the polymer film itself must be no more than about 0.005 inch thick. In order to achieve a more desirable magnetic print band, the supporting band of insulating nonmagnetic material may be replaced by a thin nonmagnetic metal support band, for example of beryllium copper.

The fluid powder bed 6 may take the form ofa plain powder box or a mechanical powdering device as, for example, any one of the powdering devices outlined in Us. Pat. application Ser. No. 770,475, filed Oct. 25, I968.

As shown in the drawings according to FIGS. 2A and 2B, the powder and scavenging discs 1 and of the apparatus according to FIGS. IA and 18 may be replaced by bands .29 and 1d which are respectively guided by rollers 19, Na and 20, 20A and respectively driven by means of the rollers I9 and 20 which are both keyed to a common drive shaft 21 connected to the drive shaft of a drive motor 25 via bevel gears 2.3 and 2d. Two members 26 located and secured one at each end of the fluid powder bed 6 and a member 22 located and screwed at the center of the fluid powder bed between the rollers 19, 19A and 20, 20A provide as shown in the drawings the bearing means for the rollers 19', 19A, 20 and 20A and allow the rollers to be rotated in the holes provided therein for the drive shaft 21 and the drive shafts of the rollers 19A and 26A.

The bands 29 and it] during the operation of the apparatus both rotate in close proximity to, and at right angles to the recording surface 3 of the horizontal magnetic print band which is being moved by means not shown in the drawings in the direction of the arrow B and performs the same function as the discs 11 and d of FlGS. RA and B.

The scavenging band 18 may either be of a permanent mag netic plastics material recorded with a series of striations, with the striations, for example substantially parallel to either the longitudinal side edge of the band or the rotational axis of the rollers l9, 19A, 20 and 20A, or a thin nonmagnetic band hav ing a keying surface for the powder. As with the arrangement according to FIGS. llA and 118, the thin nonmagnetic scavenging band would be used with the erase head l6 so that continuous or selective erasure of the magnetic pattern on the recording surface can be achieved. In the case of selective erasure, the erase head to would be repositioned such that it is situated, as shown in the dotted detail in FIGS. 2A and 2B, in the space at the center of the band 18 and positioned such that it is directly opposite the recording surface 3.

The lower end of the band 18 is positioned such that it isjust clear of the surface of the magnetic powder 49 contained within the fluid bed 6 and supported on a diaphragm 1'7, and when each section of the band 18 emerges from the fluid bed 6 the powder thereon is removed by a stripper or dressing blade 2% which is secured to the sidewall of the fluid bed 6. Alterna' tively the stripper or dressing blade 23 may be replaced by a brush attachment. It should however be noted that the lower end of the band lid may if desired be in contact with or immersed in the magnetic powder 49, the powder on the band being removed as before by a brush or a dressing blade before the outer surface of the band again encounters the recording surface 3. The powder removed by the brush or dressing blade will fall back into the fluid powder bed 6 which may take any suitable form such as hereinbefore mentioned.

The powdering band 29 is the same as the permanent erase scavenging band 18, i.e., of a permanent magnetic material recorded as before with a series of striations. The lower end of the band 29 just contacts the surface of the magnetic powder 49 contained within the fluid bed 6, and the band 29 is so positioned with respect to the magnetized recording surface 3 that powder is transferred satisfactorily to the magnetic pattern formed on the recording surface 3 by the recording head assembly T2.

The rollers 19, 19A, 20 and 20A are constructed as shown in the drawing according to FIG 3 in order to overcome the effects of powder buildup on the inside of the bands as would be the case if plain rollers were used. Referring to FIG. 3, a section through X---)( of the combined magnetic powder ap plicator and scavenging device shown in the drawing according to FIG. 2A is diagrammatically illustrated, and it can be seen that the center section of the roller 19A, around part of which the band 29 is wrapped, comprises a number of rods 33 which are secured between the flanged ends of the roller 19A such that the axes of the rods are normal to the major surfaces of the flanged ends, spaced the same distance from the central axis of the roller and equally spaced one from the other. With this arrangement, which is typical for each of the rollers, the area of the inside surface of the band which is in contact with the roller is at a minimum and the powder buildup on the in side surface of the band is considerably reduced since any powder that collects on the inside surface falls as the roller is rotated through the spaces between the rods 33 into the fluid bed 6, therefore the clearance between the bands 18 and 29 and the recording surface 3 which is critical for correct operation is tended to be maintained at the desired value.

The combined powder applicator and scavenging devices shown in the drawings according to FIGS. llA, TB, 2A and 2B may be adapted to powder and scavenge a plurality of horizontal magnetic print bands which are arranged one above the other to form individual displays or one large display, merely by extending the length of the bands 29 and 118 or increasing the diameter of the discs l and 5. it should however be noted that from a practical standpoint the combined powder applicator and scavenging device according to FIGS. 2A and 2B is more adaptable to this arrangement.

The combined powder applicator and scavenging device according'to FIGS. 2A and 28 may be modified, as shown in the drawing according to FIG. 4, to powder and scavenge a vertical magnetic print band 36 which is guided by the rollers 37 and driven in the direction of the arrow C by means not shown in the drawing.

In this arrangement the scavenging band 40 and its guide rollers 41 are respectively constructed and operate in the same manner as the scavenging band 113 and the rollers 19, 19A, and 20A, according to FIGS. 2A and 2B. The erase head 16 is utilized as before when a thin nonmagnetic scavenging band is used or when the permanently magnetized scavenging band does not effectively erase the magnetic pattern formed on the recording surface 3 by the recording head assembly 12. The powder on the scavenging band is removed by cleaning means such as a dressing blade 42, which may be replaced by a brush attachment.

A rotatably mounted roller 43 is the powder applicator, and the surface 44 thereof is covered with a thin sheet of permanently magnetized material or the roller 43 itself may be provided by a permanent magnet. The roller 43, which is partially immersed in the magnetic powder 49 within the fluid bed 6, operates in exactly the same manner as the powdering band 29 according to FIGS. 2A and 2B, and the same conditions apply in relation to the position of the surface 44 relative to the recording surface 3 in order to achieve satisfactory powder transfer.

The device shown in the drawing according to FIG. 4 may also be adapted to powder and scavenge a plurality of vertical magnetic print bands merely by extending the width of the band 40 and the length of the roller 43 to suit the particular requirement.

The main advantages of the combined powder applicator and scavenging devices outlined in the preceding paragraphs, are that they are easily removed from the remainder of the apparatus, thereby facilitating maintenance and adjustment, and the scavenged powder falls back into the fluid bed, thereby providing a lossless powder system for the apparatus. Also the devices are arranged such that they allow the recording head assembly and powdering station to be close to the viewing position, therefore the time which elapses between recording and viewing of a character is at a minimum.

It should be noted that if the band 36 was driven in a direction opposite to the direction indicated by the arrow C, and the erase head 16 was relocated on the inside surface of the band 36 to a position between the recording head assembly 12 and the roller 37 which is adjacent to the roller 43, then the band 40 would become the powder applicator and the roller 43 the powder scavenging device.

The powder formulation which is utilized in the apparatus outlined in the preceding paragraphs to develop the magnetic pattern formed by the recording head assembly 12 on the recording surface 3 includes a magnetic powder such as magnesium-nickel ferrite or nickel-zinc ferrite which have a dielectric property that may be utilized in a combined powder applicator and scavenging device. For example, the pickup surface of the powdering disc 1, belt 29 or 40 or roller 43 may be arranged such that it can be charged electrostatically, therefore the magnetic powder will be attracted to the pickup surface to form an evenly distributed layer due to its dielectric property. With this arrangement the problems which arise due to the demagnetizing effects on the latent magnet image or images by a permanently magnetized pickup surface no longer exist and the displacement of the pickup surface relative to the recording surface 3 is no longer critical. This arrangement may also be adopted for the scavenging disc, belt or roller as the case may be.

It should be noted that when the pattern on the recording surface of the print band is formed electrostatically the recording surface 3 will be of a material such as a dielectric material nd the powder formulation will include a dielectric powder or ferroelectric powder, for example barium titanate, to form the image or pattern. Therefore, with this arrangement of the apparatus the magnetic or dielectric property of the powder may be utilized to effect the combined powder application and scavenging.

Iclaim:

1. Apparatus of the type in which charge-attractable powder in a powder reservoir is applied to and scavenged from a movable recording surface having charge patterns recorded thereon, comprising a combined powder applicator and scavenging device including a pair of movable surfaces which are spaced apart in the direction of movement of the recording surface and which are parallel with the recording surface for transferring powder between the reservoir and the recording surface, at least one of said pair of movable surfaces being permanently charged and partially submerged in the powder contained in the reservoir.

2. Apparatus as claimed in claim 1 wherein the recording surface is provided by one side of a magnetic tape.

3. Apparatus as claimed in claim 2 wherein the magnetic tape is arranged to move in a horizontal plane relative to said combined powder applicator and scavenging device.

4. Apparatus as claimed in claim 1 wherein each of said pair of movable surfaces is provided by a rotatable disc having a flat face.

5. Apparatus as claimed in claim 4 wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization of the flat face of the one rotatable disc takes the form of a series of striations.

6. Apparatus as claimed in claim 5 wherein said series of striations take the form of radials from the center of said rotatable disc.

7. Apparatus as claimed in claim 6 wherein at least one of the rotatable discs is a permanent disc magnet.

8. Apparatus as claimed in claim 6 wherein at least one of the rotatable discs is nonmagnetic with a layer of permanently magnetized material on the face thereof.

9. Apparatus as claimed in claim 6 wherein one of the rotatable discs is permanently charged and the flat face of the other disc is arranged to provide a keying surface for the powder.

10. Apparatus as claimed in claim 6 wherein a predetermined portion of the flat face of each of said rotatable discs at any one time scans the recording surface to respectively apply powder thereto and scavenge powder therefrom.

ill. Apparatus as claimed in claim 6 further including singledrive means to driven both of said discs.

12. Apparatus as claimed in claim 3 wherein each of said pair of movable surfaces is provided by one side of a flexible band, and wherein said flexible bands are guided and driven by means of a series of rollers.

13. Apparatus as claimed in claim 12 wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization thereof takes the form of a series of striations.

I4. Apparatus as claimed in claim 12 wherein said series of striations are substantially parallel to the longitudinal side edge of said flexible bands.

15. Apparatus as claimed in claim I4 wherein said at least one magnetized surface is made of a permanent magnetic plastics material.

I6. Apparatus as claimed in claim 14 wherein one of the flexible bands is permanently magnetized and the other flexible band is arranged such that it provides a keying surface for the powder.

17. Apparatus as claimed in claim 14 wherein a predetermined portion of the flexible bands at any one time scans the recording surface to respectively apply powder thereto and scavenge powder therefrom.

18. Apparatus as claimed in claim I4 further including single drive means for driving both of said flexible bands, and coupled to one of the series of rollers.

19. Apparatus as claimed in claim 2 wherein the recording surface is arranged to move in a substantially vertical plane relative to said combined powder applicator and scavenging device.

20. Apparatus as claimed in claim 11 wherein one of said pair of movable surfaces is provided by one side of a flexible band which is guided and driven by means of a series of rotatable rollers, and wherein the other said pair of movable surfaces is provided by the surface of a rotatable drum.

21. Apparatus as claimed in claim wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization thereof takes the form of a series of striatrons.

22. Apparatus as claimed in claim 21 wherein the flexible band is made of a permanent magnetic plastics, and wherein the drum is comprised of a layer of permanently magnetized material on the surface thereof.

23. Apparatus as claimed in claim 20 wherein one movable surface is arranged to provide a keying surface for the powder.

24. Apparatus as claimed in claim 2 further including cleaning means for removing powder from one of said pair of movable surfaces.

25. Apparatus as claimed in claim 24 wherein said cleaning means are provided by a dressing blade located within and secured to said fluid reservoir.

26. Apparatus as claimed in claim 12 wherein the rollers include a plurality of rods secured between two flange members such that the axes of the rods are normal to the major surfaces of said flange members, are spaced the same distance from the central axis of said roller and are equally spaced one from the other.

27. Apparatus as claimed in claim 2 further including a magnetic erase unit for erasing the charge images recorded on said magnetic tape.

28. Apparatus as claimed in claim 27 wherein said magnetic erase unit is an electromagnetic.

29. Apparatus as claimed in claim 28 wherein the powder reservoir is provided by a fluid powder bed. 

1. Apparatus of the type in which charge-attractable powder in a powder reservoir is applied to and scavenged from a movable recording surface having charge patterns recorded thereon, comprising a combined powder applicator and scavenging device including a pair of movable surfaces which are spaced apart in the direction of movement of the recording surface and which are parallel with the recording surface for transferring powder between the reservoir and the recording surface, at least one of said pair of movable surfaces being permanently charged and partially submerged in the powder contained in the reservoir.
 2. Apparatus as claimed in claim 1 wherein the recording surface is provided by one side of a magnetic tape.
 3. Apparatus as claimed in claim 2 wherein the magnetic tape is arranged to move in a horizontal plane relative to said combined powder applicator and scavenging device.
 4. Apparatus as claimed in claim 1 wherein each of said pair of movable surfaces is provided by a rotatable disc having a flat face.
 5. Apparatus as claimed in claim 4 wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization of the flat face of the one rotatable disc takes the form of a series of striations.
 6. Apparatus as claimed in claim 5 wherein said series of striations take the form of radials from the center of said rotatablE disc.
 7. Apparatus as claimed in claim 6 wherein at least one of the rotatable discs is a permanent disc magnet.
 8. Apparatus as claimed in claim 6 wherein at least one of the rotatable discs is nonmagnetic with a layer of permanently magnetized material on the face thereof.
 9. Apparatus as claimed in claim 6 wherein one of the rotatable discs is permanently charged and the flat face of the other disc is arranged to provide a keying surface for the powder.
 10. Apparatus as claimed in claim 6 wherein a predetermined portion of the flat face of each of said rotatable discs at any one time scans the recording surface to respectively apply powder thereto and scavenge powder therefrom.
 11. Apparatus as claimed in claim 6 further including single-drive means to drive both of said discs.
 12. Apparatus as claimed in claim 3 wherein each of said pair of movable surfaces is provided by one side of a flexible band, and wherein said flexible bands are guided and driven by means of a series of rollers.
 13. Apparatus as claimed in claim 12 wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization thereof takes the form of a series of striations.
 14. Apparatus as claimed in claim 12 wherein said series of striations are substantially parallel to the longitudinal side edge of said flexible bands.
 15. Apparatus as claimed in claim 14 wherein said at least one magnetized surface is made of a permanent magnetic plastics material.
 16. Apparatus as claimed in claim 14 wherein one of the flexible bands is permanently magnetized and the other flexible band is arranged such that it provides a keying surface for the powder.
 17. Apparatus as claimed in claim 14 wherein a predetermined portion of the flexible bands at any one time scans the recording surface to respectively apply powder thereto and scavenge powder therefrom.
 18. Apparatus as claimed in claim 14 further including single drive means for driving both of said flexible bands, and coupled to one of the series of rollers.
 19. Apparatus as claimed in claim 2 wherein the recording surface is arranged to move in a substantially vertical plane relative to said combined powder applicator and scavenging device.
 20. Apparatus as claimed in claim 11 wherein one of said pair of movable surfaces is provided by one side of a flexible band which is guided and driven by means of a series of rotatable rollers, and wherein the other said pair of movable surfaces is provided by the surface of a rotatable drum.
 21. Apparatus as claimed in claim 20 wherein at least one of said pair of movable surfaces is charged magnetically such that magnetization thereof takes the form of a series of striations.
 22. Apparatus as claimed in claim 21 wherein the flexible band is made of a permanent magnetic plastics, and wherein the drum is comprised of a layer of permanently magnetized material on the surface thereof.
 23. Apparatus as claimed in claim 20 wherein one movable surface is arranged to provide a keying surface for the powder.
 24. Apparatus as claimed in claim 2 further including cleaning means for removing powder from one of said pair of movable surfaces.
 25. Apparatus as claimed in claim 24 wherein said cleaning means are provided by a dressing blade located within and secured to said fluid reservoir.
 26. Apparatus as claimed in claim 12 wherein the rollers include a plurality of rods secured between two flange members such that the axes of the rods are normal to the major surfaces of said flange members, are spaced the same distance from the central axis of said roller and are equally spaced one from the other.
 27. Apparatus as claimed in claim 2 further including a magnetic erase unit for erasing the charge images recorded on said magnetic tape.
 28. Apparatus as claimed in claim 27 wherein said magnetic erase unit is an electromagnetic.
 29. Apparatus as claimed in claim 28 wherein the powder reservoir is provided by a fluid powder bed. 